1. Technical Field
The invention relates to a disc brake for a heavy vehicle comprising a disc brake having a disc-shaped rotor consisting of a cast iron alloy and a calliper supporting a brake lining which is intended to be pressed against the said rotor during braking. More particularly, the invention relates to disc brakes intended to absorb a braking torque equal to and greater than 12 kNm, especially a braking torque between 12 and 25 kNm. These brakes are intended for heavy vehicles, by which is meant vehicles having an axle pressure in excess of 5 tons, especially vehicles having an axle pressure between 6 and 14 tons.
2. Background Art
Disc brakes are being used more and more frequently for heavy vehicles. A distinction can be drawn between two types of disc brakes for heavy vehicles, firstly hat-shaped brake discs in which a disc with radial extent is configured with a centrally situated, axially running cylindrical protrusion intended for fastening to a wheel axle, and secondly brake discs which are substantially symmetrical with respect to a plane running at right angles through the rotational axis of the brake disc and have a central bushing intended for fastening to a wheel hub. One problem with hat-shaped brake discs is that the brake disc is not supported symmetrically with respect to a plane running at right angles through the rotational axis of the brake disc, which means that the disc slants when it is heated. The effect of this is that the pressure from the brake lining is not evenly distributed over the brake, so that the brake disc is at greater risk of cracking. In order to reduce the risk of disc fracture, brake discs have therefore been produced which are substantially symmetrical with respect to a plane running at right angles through the rotational axis of the brake disc and which have a central bushing intended for fastening to a wheel hub. Such a brake disc is shown in EP 621 829. The bearing surfaces of these discs remain flat during heating, so that the pressure from the brake linings is more advantageously distributed and the risk of cracking is reduced.
A further problem with the use of brake discs is the occurrence of bandwear. Band-shaped wear is a phenomenon which occurs as a result of the contact surface between lining and rotor not being completely flat. This phenomenon is explained in greater detail below in connection with FIGS. 1a-1d of the drawing. FIG. 1a shows a disc brake 1 in cross section, in which the disc brake 1 comprises a brake lining 32 and a disc-shaped rotor 8. The brake lining 32 has a contact surface 38, which faces a corresponding contact surface 39 of the rotor 8. FIG. 1a shows a disc brake having perfectly flat surfaces. All friction pairs have, however, certain irregularities. FIG. 1b shows, on an exaggerated scale, a brake lining and a rotor which have an irregularity, contact taking place within a first region 40. The region forms a band which runs around the disc. When braking occurs, a higher pressure is generated within this region, so that the majority of the braking energy is absorbed within the region. This means that the region is heated more than adjoining regions. The effect of this is that the lining and the rotor expand, whereupon an even higher pressure is developed and the irregularity is enlarged, as shown in FIG. 1c. The first region will suffer greater wear, however, than surrounding regions, firstly because the friction pair is warmer in this region, secondly because the contact pressure is heavier. This results in the irregularity being worn down and the place of contact being shifted to another place on the brake disc, as shown in FIG. 1d. 
The presence of bands gives rise to the formation of local plasticized zones, thereby producing an increased risk of cracking. The successive formation of bands to and fro between central and peripheral regions of the disc further gives rise to the risk of fatigue failure as a result of the constantly occurring load change. It has been shown that the band can move one or more times between outer edges and central sections of the disc, even during one and the same braking cycle.
One object of the invention is to provide a brake disc offering reduced occurrence of disc cracking. Another object of the invention is to provide a brake disc in which the frequency of change between different band modes is reduced.